Figuring out Fall Foliage

If you happen to go outside this month, you might notice that the trees around town are doing something predictable, yet strange: they're changing colors! Leaves fade from brilliant green to red, orange, and yellow, then droop to brown as they litter the ground. This happens ever year, but why? Where do the colors come from?

As winter approaches, the nights get longer. That signals trees to start shedding their leaves, which (unlike the twigs, branches, and trunks) aren't adapted to withstand the cold of winter. To make that happen, the tree gradually cuts off the supply of fluids to the leaves. Without a constant supply of sugars and sap, the chemicals in the plant start to change.

First, chlorophyll decays. Chlorophyll is a chemical that absorbs sunlight and turns it into energy through a process called photosynthesis; it's also what gives leaves their green color. During the spring and summer months, the plant produces lots of chlorophyll, but in the fall production slows down and what's there starts to fade.

As the amount of chlorophyll in a leaf decreases, other colors that were hidden in the leaf all along start to show up. These are the yellow and gold hues, and they're caused by a set of chemicals called carotenoids. At the same time, another set of chemicals, anthocyanins, turn the leaves red and crimson as they're produced in the leaves.

Eventually, the leaves, cut off from the fluids in the rest of the tree, dry up and fall off. They'll provide food and insulation for the tree as they decompose around it. The tree is now ready for winter!